Models for the ultimate development of effective vaccines and immunotherapies that would limit HIV replication can be drawn from naturally occurring examples of immune system-mediated control. Identifying the components, targets, and magnitude of an effective immune response to HIV are important steps toward developing effective vaccines and immunotherapies. Although patients with normal CD4+ T cell counts and low levels of plasma virus are a heterogeneous group, a small subgroup of patients with truly non-progressive HIV infection and restriction of virus replication likely holds important clues to the basis of an effective immune response to HIV. A small subpopulation of HIV-infected individuals (fewer than 0.8%) shows no signs of disease progression over a 10-year period. We have assembled a stringently defined cohort of such patients, termed long-term nonprogressors (LTNPs), or elite controllers. Many of these patients have been infected for 20 years, yet even without receiving antiretroviral therapy, they have experienced no CD4+ T-cell decline and have maintained plasma viral RNA levels below 50 copies per milliliter. We are using cells from these patients to systematically dissect the mechanisms of immune-mediated restriction of virus replication. The HIV-specific T-cell responses of these patients have been studied in extreme detail. Through this project, extraordinary progress has been made in understanding how the immune system controls HIV. Our prior work indicated that there is a dramatic association between immunologic control and the HLA B*5701 allele, and that the immune response is highly focused on peptides restricted by this allele. This result established both host genetic and functional links between immunologic control and the CD8+ T-cell responses of these patients. More recently, we have found that this focus is specific to HIV and is not found in the response to other pathogens such as hepatitis C virus or cytomegalovirus. LTNPs and progressors do not differ in the frequency of HIV-specific T cells or in the ability to recognize the autologous virus. The finding of high frequencies of CD8+ T cells specific for the patients virus in both LTNPs and progressors strongly suggested that differences between responses of these patient groups were qualitative rather than quantitative in nature. One important qualitative difference in the HIV-specific immune response that distinguishes LTNPs from progressors is the maintenance of HIV-specific CD8+ T cells with a high proliferative capacity. This proliferation parallels perforin expression required for effective killing of HIV-infected CD4+ T cells. We have recently established the properties of the HIV-specific CD8+ T-cell response that are tightly associated with the LTNP phenotype. Although the HIV-specific CD8+ T cells of LTNPs have a greater capacity to proliferate and increase their number of molecules responsible for killing HIV-infected cells, the mechanism(s) by which these properties translate into effective immunologic control of HIV has remained unknown. Most current assays are not sufficiently powerful to establish if differences in HIV-specific CD8+ T-cell function are determined by frequency, CD8+ T-cell proliferation, preferential target or effector cell death, or the mechanism of HIV-infected cell elimination. To better understand the mechanisms of immunologic control, we have recently devised a method to measure HIV-infected cell elimination on a per-cell basis. Measured on a per-cell basis, HIV-specific CD8+ T cells of LTNPs efficiently eliminated primary autologous HIV-infected CD4+ T cells. This effective killing was clearly distinguishable from the responses of progressors over a very broad range of effectors to HIV-infected targets. Progressor cells did not mediate effective killing even at high effector-to-target ratios. Defective cytotoxicity of progressor effectors could be restored in vitro. These results establish an effector function and a mechanism that clearly segregate with immunologic control of HIV. We recently extended our earlier work and comprehensively examined a broad array of functions of HIV-specific T-cells derived from two large patient groups, LTNPs and progressors on antiretroviral therapy (Rx<50), who possess comparable levels of HIV viremia as determined by a sensitive single-copy assay. In response to autologous HIV-infected CD4+ T cells, HIV-specific CD8+ T cell proliferative capacity, IL-2 responsiveness, surface phenotype, PD-1 expression, polyfunctionality, and cytotoxic capacity are measured in considerable detail. We observe that HIV-specific CD8+ T cell polyfunctionality, IL-2 responsiveness, and proliferative and killing capacities are not restored by antiretroviral therapy. A deeper understanding of the basis of immunologic control in LTNPs and the loss of immunologic control in progressors is likely to provide information that is critical for developing immunotherapies or prophylactic vaccines for HIV. In future work, we will seek to understand the molecular basis of the difference in killing capacity between LTNPs and progressors. We are also working to understand the mechanism by which such responses arise in LTNP patients, and to determine whether these responses may be exploited in an HIV vaccine. Although cytotoxic capacity clearly distinguishes those individuals with immunologic control in the setting of chronic infection, it may not necessarily be the operative mechanism in control induced by an HIV vaccine. Thus far, similar measurements of recall cytotoxic capacity against HIV-infected CD4+ T cells have not yet been applied to recipients of HIV vaccines. Our laboratory is vigorously pursuing an understanding of the cytotoxic capacity induced by recently tested HIV vaccines.